1. Field of Invention
The present invention relates to heat-treating, and more particularly to systems and methods for supporting a workpiece during heat-treating thereof.
2. Description of Related Art
Numerous applications involve heat-treating a workpiece. For example, in the manufacture of semiconductor chips such as microprocessors, the workpiece typically includes a semiconductor wafer, supported in a thermal processing chamber for annealing or other heat-treating purposes. Commonly owned U.S. patent application Ser. No. 11/018,388 (publication no. US 2005/0133167, hereby incorporated herein by reference) discusses examples of heat-treating techniques for annealing such semiconductor wafers, in which the wafer is first pre-heated to an intermediate temperature, following which the top or device side surface is rapidly heated to an annealing temperature. The initial pre-heating stage occurs at a rate significantly slower than a thermal conduction time through the wafer, and may be achieved by irradiating the back-side or substrate side of the wafer with an arc lamp or other irradiance device, to heat the wafer at a ramp rate less than 400° C. per second, for example. The subsequent surface heating stage occurs much more rapidly than the thermal conduction time through the wafer, so that only the device side surface is heated to the final annealing temperature, while the bulk of the wafer remains close to the cooler intermediate temperature. Such surface heating may be achieved by exposing the device-side surface to a high-power irradiance flash from a flash-lamp or bank of flash lamps, the flash having a relatively short duration, such as one millisecond, for example. The cooler bulk of the wafer then acts as a heat sink to facilitate rapid cooling of the device side surface.
Such annealing methods, which involve rapidly heating the device side of the wafer to a substantially higher temperature than the bulk of the wafer, tend to cause the device side to thermally expand at a greater rate than the rest of the wafer. Depending on the magnitude of the temperature difference between the device side temperature and the temperature of the bulk of the wafer, this may tend to cause “thermal bowing”, whereby the normally planar wafer deforms itself into a thermally deformed shape. Depending on the magnitude and rapidity of the device side heating stage, the thermally deformed shape may have attributes of a dome shape, with the center of the wafer tending to rapidly rise relative to its edge regions. The thermal bowing may also cause the outer perimeter or edge of the workpiece (such as the outer two or four centimeters of a 30-cm diameter wafer, for example) to curl downward steeply, and thus, the thermally deformed shape may also have attributes of a saucer shape similar to a FRISBEE™ flying disc. In practice, for some applications it has been found that the latter curling effect at the outer perimeter of the workpiece tends to be more pronounced than the former dome-shaped curvature of the workpiece as a whole, although this may depend on the physical parameters of the thermal cycle in question. The thermally deformed shape represents a reduced stress configuration of the wafer, lowering the thermal stress resulting from the temperature gradient between the device side and the bulk of the wafer, and it is therefore undesirable to rigidly prevent this thermally induced motion.
Due to the extreme rapidity at which the device side of the wafer is heated (in the course of a 1-millisecond flash, for example, much faster than a typical thermal conduction time in the wafer), the deformation of the wafer may occur sufficiently rapidly that the edges of the wafer tend to move rapidly downward. If the wafer is supported by conventional support pins near its edges, the thermal bowing of the wafer may apply large downward forces to the support pins, potentially damaging or destroying both the pins and the wafer. Such forces may also cause the wafer to launch itself vertically upward from the support pins, which may result in further damage to the wafer as the wafer falls back down and strikes the pins. If the wafer is supported by support pins located further radially inward, the edges of the wafer may rapidly bow downward and strike a support plate above which the wafer is supported, potentially damaging or destroying the wafer. In addition, due to the rapidity at which such thermal bowing occurs, the initial velocities imparted to the various regions of the wafer tend to cause the wafer to overshoot the equilibrium minimum stress shape and rapidly oscillate or vibrate, resulting in additional stress and potentially damaging or destroying the wafer.
The above-noted US patent application publication no. US 2005/0133167 discloses, among other things, methods and apparatuses for suppressing thermally induced motion of a workpiece. In one such method, a damping member is spaced apart from the workpiece and is configured to apply a damping force to dampen the motion of the workpiece. In a particular illustrative embodiment, the damping member includes a quartz plate, the workpiece includes a semiconductor wafer, and the quartz plate is spaced apart from an initial position or rest position of the wafer by a distance sufficiently small that gas pressure between the quartz plate and the wafer opposes the thermally induced motion of the workpiece. For example, the quartz plate and the wafer may be spaced apart by only a distance of about one millimeter. In such a case, the irradiance flash may cause the central region of the wafer to rapidly rise upward away from the quartz plate, thereby creating a low gas pressure zone in the gap between the wafer and the plate. Thus, a pressure differential is created, between the higher ambient pressure above the wafer, and the lowered pressure in the gap beneath the wafer, which tends to oppose the upward motion of the wafer. Conversely, if the central region of the wafer comes back downward and overshoots its equilibrium position, this creates a higher pressure in the gap between the wafer and the plate than the ambient pressure above the wafer, so that the pressure differential continuously opposes the motion of the wafer as it oscillates or vibrates. Thus, motion and vibration of the wafer are dampened, without the necessity of potentially damaging physical contact between the wafer and the damping member.